Even Distribution after allocation

[NinaHerrmann]

This issue aim might be a simplified issue of #148

Description: As a teacher, I want to offer tutorials that have, when possible, evenly distributed members.

Steps:

1. When I create a ratingallocate, I want to enable a configuration setting (I would favour it as the default, but I do not have a strong opinion on that), enabling even distribution after allocating students to choices.
2. Students make choices as usual and are allocated to the choices available
3. afterwards (but in the same cron task), it is checked that each choice has, if possible, the same number of students. For example, if I offer a tutorial in Room 1 and a tutorial in Room 2, both at 08:00-10:00, and students rate both at the same number, those tutorials should have (if possible) the same number of participants.

In our case, we have tutorials, which are given anyway, as the Tutors are already paid. However, those have similar efforts; therefore, the tutorials should be the same size.